Bacteria convert harmful nitrogen by-products from aquatic animals into less harmful nitrates. There are some interesting articles about water chemistry in the USA Koi Blog section on the website.
This process occurs naturally in nature's lakes and streams. In a Koi pond these bacteria are found on the walls of our ponds. They also can be found on the inside walls of the pond plumbing, attached to the skimmer basket, and on the rocks that form a waterfall.
The reason it is important to consider what filter media to use when we build a biological filter systems is because different media have different amounts of surface area. The more surface area the more bacteria can colonize and live in your filter to process all of the Koi waste or un-eaten food. Because of the weight of rock it can be hard to clean and it doesn't necessarily have the most surface area of all of the different types of filter media available.
The important consideration is to keep the pollutants and unwanted debris out of our ponds, while keeping the beneficial bacteria in our systems. If you have any questions, or would like advice on which filter media is best suited to your task, please feel free to contact us for help.